The development of low-cost complementary metal-oxide semiconductor (CMOS) image sensors has contributed to the incorporation of color camera modules in high-volume consumer products such as mobile devices and motor vehicles. Advances in such camera modules include incorporation of infrared (IR) detection capability, and hence dual mode imaging of both visible and infrared spectral bands, which are important for applications such as gesture recognition and depth analysis.
Herein the terms IR light, IR electromagnetic radiation, and IR wavelengths refer to electromagnetic energy at wavelengths between λ≈0.75 μm and λ≈1.1 μm. The upper limit of λ≈1.1 μm corresponds to the band gap energy of silicon in CMOS image sensors. Similarly, the terms visible light, visible electromagnetic radiation, and visible wavelengths refer to electromagnetic energy at wavelengths between 0.40 μm and 0.75 μm.
FIG. 1 shows a camera module 120 of a mobile device 190. FIG. 2 is a cross-sectional view of camera module 120. Camera module 120 includes an imaging lens 222 and image sensor 250 having a pixel array 200. Imaging lens 222 is capable of imaging light 232 propagating from an object 234 onto pixel array 200. Image sensor 250 may include a device die 206 that is, for example, implemented using CMOS processes, but may be implemented in other technologies without departing from the scope hereof. Device die 206 is electrically connected to a printed circuit board (PCB) 230.
FIG. 3 is a cross-sectional view of a prior-art IR-sensitive pixel 300 compatible for use in pixel array 200. IR-sensitive pixel 300 includes an IR band-pass filter 374 between a microlens 376 and a silicon substrate 320. Substrate 320 includes photodiode regions 322 and is formed of a semiconductor such as silicon. Substrate 320 may include layers and regions of different materials without negating its function as a substrate for filter 374.
A problem with IR-sensitive pixel 300 is that silicon substrate 320 absorbs infrared light much less efficiently that it does visible light. The electric field of light incident on a medium surface decays within the medium to e−1 of its initial value at a distance
      z    ⁡          (      λ      )        =      λ          2      ⁢              πκ        ⁡                  (          λ          )                    from the surface, where K(λ) is the imaginary part of the medium's refractive index at wavelength λ. For silicon, κ(λ) at λ=500 nm (green light) is more than ten times that of κ(λ) at λ=830 nm (near-IR), which means that, compared to visible light, IR light propagates roughly ten times further into silicon before being absorbed. This decreased absorption of IR light limits the sensitivity of pixel array 200 to IR light as compared to visible light.